1. Field of the Invention
The present invention relates to an optical element which can be suitably used for recording/reproducing information using an optical information recording medium such as a video disk, a compact disk, an optomagnetic recording apparatus or the like.
2. Description of the Prior Art
An optical element used for the purpose described above focuses parallel light from a light source onto a recording medium. Aberration of the element outside the optical axis, especially coma, must be sufficiently corrected in consideration of an assembly error introduced by assembly of an optical system and application conditions, e.g., a plurality of light beams are passed through a single optical element and a tracking beam among these incident beam is incident at an element position outside the optical axis. In practice, when the wavelength of light used is represented by .lambda., the residual wavefront aberration must be within .lambda./4. In addition, the element must be compact in size, light in weight, and inexpensive. In the past, various types of optical information recording/reproducing optical elements have been proposed. However, each of such elements is subject to some problems.
For example, when a focusing lens consisting of a number of normal spherical lenses each comprising a transparent material having a uniform refractive index is used, since the lens becomes large in size and heavy, the overall apparatus cannot be rendered compact in size and light in weight. In addition, due to the heavy weight of the lens, tracking and servo control of the optical pickup is adversely affected.
When light beams are passed through a number of lenses, reflection loss occurs at each lens surface, and the optical power of light reaching the recording medium is reduced. For this reason, beam control reliability is impaired.
In addition, since the optical system becomes complex in structure, high-precision processing and assembly cannot be performed.
In order to solve these problems encountered with an assembled spherical lens, a graded index lens having one concave surface has been proposed as in Japanese Patent Disclosure No. 58-122512. However, in general, an optical information recording/reproducing element has a very small diameter below several millimeters, for example. Therefore, it is very difficult to process one surface into a concave surface having a predetermined radius of curvature. Even if a concave surface with a predetermined radius of curvature can be obtained, a sharp edge of the lens is subject to chipping. Furthermore, with a lens having a flat surface at the light source side and a concave surface at the recording medium side, since the concave surface has a negative power, a desired NA cannot be obtained unless a very large power is obtained with a refractive index distribution within the lens. This requires a large difference in refractive index within the lens, and control of terms of high orders of a refractive index constant, rendering the manufacture of the lens extremely difficult.